Hot cathode ion sources are well known and include for example the so called Freeman ion source and the so called Bernas ion source. These ion sources include a filament cathode which is directly heated by passing a current through the filament from a filament power supply. The ion source comprises an arc chamber containing the filament and to which a supply of gas or vaporised material is supplied. Once the cathode filament has been heated sufficiently with the filament current, thermionic electrons are emitted by the cathode. If the cathode is held at a substantial negative potential relative to an anode electrode in the arc chamber, a plasma is formed in the arc chamber with an arc current flowing between the cathode and the anode electrode. Typically, the anode electrode is in fact formed by the walls of the arc chamber.
In the plasma so formed in the arc chamber, molecules of the feedstock gas or vapour are ionised and these positive ions are extracted from the arc chamber through an aperture by an extraction electrode held at a negative potential relative to the arc chamber. The extracted ions may then be used to form an ion beam which may have a number of applications. One important application is in ion beam implantation where beams of ions of desired dopant materials are directed at semiconductor substrates (wafers) in order to implant the dopant in the semiconductor to provide desired conductivity conditions.
A Freeman type ion source, especially for ion beam implantation apparatus, is disclosed in U.S. Pat. No. 4,578,589. The Freeman ion source includes a filament power supply to provide a heating current through the cathode filament, and an arc power supply. U.S. Pat. No. 4,754,200 discloses a method of controlling the power supplies to optimise performance of the ion source, especially in its application in an ion implantation apparatus.
U.S. Pat. No. 5,262,652 discloses a Bernas-type ion source in an ion implanter application. Again the Bernas-type source has a directly heated cathode filament with a filament power supply to provide a heating current through the filament, and a separate arc power supply to provide a desired arc potential between the filament and the anode or arc chamber body.
The above mentioned U.S. Pat. No. 4,754,200 discloses circuits for controlling the ion source power supplies. Thus, it is known to apply a constant arc voltage between the filament and the arc chamber body (or anode) and then to adjust the filament supply to achieve a desired arc current. It is also known to control the filament power supply by means of a power demand signal, i.e. to achieve an output power from the filament power supply in accordance with an input power demand signal, where the filament input power demand is derived from an error in the arc voltage, the arc current being held constant.
U.S. Pat. No. 5,497,006 discloses an ion source of the Bernas-type, but with an indirectly heated cathode. In this arrangement, the cathode in the arc chamber of the source is formed of an electrically conductive button-like member which is indirectly heated from a separate filament located behind the button on the opposite side relative to the main plasma chamber of the ion source. The source includes not only an arc power supply and a filament power supply, but also a cathode bias power supply which provides a required bias potential between the filament and the cathode button. In operation, the filament is biased negatively relative to the cathode button so that electrons thermionically emitted by the filament are accelerated to strike the rear face of the button, thereby heating the button cathode so that this cathode in turn emits electrons into the plasma chamber to initiate or maintain the plasma arc of the source.
This arrangement with an indirectly heated cathode in the ion source greatly extends the life of the ion source before it is necessary to change either the cathode itself or the cathode heater filament. The above form of ion source with an indirectly heated cathode will be referred to hereinafter as an indirectly heated cathode-type ion source as hereinbefore defined.
U.S. Pat. No. 5,497,006 discloses an ion source having a filament power supply unit, a cathode bias power supply unit and an arc power supply unit, and a power control apparatus for controlling these power supplies. In the U.S. specification, programmable power supply units are used for each of these supplies. The arc power supply unit is controlled to provide an output arc voltage in accordance with an input voltage demand level. The bias voltage applied between the cathode button and the filament by the cathode bias power supply unit is set in accordance with the difference between a desired arc current and the measured current delivered to the arc by the arc power supply, so as to minimise this difference. Thus, if the arc current is below the demand value, the cathode bias supply voltage is increased to increase the heating energy delivered to the cathode button, thereby to reduce the impedance of the plasma in the arc chamber and consequently to increase the arc current. The filament power supply in U.S. '006, is in turn controlled to keep the current delivered by the cathode bias power supply equal to a desired level of current. Thus, the current from the cathode bias power supply is maintained at a required demand level by increasing or decreasing the voltage applied by the filament power supply to the filament.